IV INTRODUCTION. 



into rocky strata, which, as time rolled on, were again raised to 

 form plains, hills and mountain chains, subject to the same 

 ceaseless round of destruction and re-formation to which they 

 owed their origin. 



But the materials of which even our hardest stratified rocks 

 are composed were not, in every instance, entirely and directly 

 derived from the wreck of pre-existing solid rocks, for there 

 was some working over of the same materials, without the action 

 of the intervening process of consolidation ; and the remains of 

 corals, crinoids, shells, and other marine organisms, often con- 

 tributed much the larger portion of the substances composing 

 marine strata, while in other instances the remains of land and 

 fresh-water animals and plants, carried in by streams, form con- 

 siderable portions of extensive estuary and lacustrine formations. 

 Hence, as the animals and plants of each of the great epochs, 

 although mainly different from those of the preceding and fol- 

 lowing ages, presented, with occasional exceptions, a remarkable 

 general similarity during each individual period, over the entire 

 globe, their fossil remains furnish the Geologist an unerring 

 guide in the identification of strata, and the determination of 

 their position in the series, as well as of their relative ages. As 

 the learned Antiquary determines the age and history of some 

 ruined city, in regard to which both written and traditional 

 history are silent, by a careful study of the inscriptions upon its 

 walls, or of the seals, coins, medals, etc., found amongst its rub- 

 bish, so does the skillful Paleontologist determine the period 

 of the earth's history to which an outcrop or stratum of rock 

 belongs, and its place in the geological series, by inspecting its 

 imbedded organic remains. 



When it is therefore borne in mind, that coal and other valua- 

 ble minerals were not indiscriminately distributed through the 

 earth, but were mainly formed or deposited, at least in quantities 

 and under conditions to be useful to man, during particular 

 geological periods, the importance of knowing to what epoch of 





